Spacers with Rectangular Profile and Methods of Forming the Same

ABSTRACT

A method includes forming a spacer layer on a top surface and sidewalls of a patterned feature, wherein the patterned feature is overlying a base layer, A protection layer is formed to contact a top surface and a sidewall surface of the spacer layer. The horizontal portions of the protection layer are removed, wherein vertical portions of the protect layer remain after the removal. The spacer layer is etched to remove horizontal portions of the spacer layer, wherein vertical portions of the spacer layer remain to form parts of spacers.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No. 14/097,579, entitled “Spacers with Rectangular Profile and Methods of Forming the Same,” filed on Dec. 5, 2013, which application is incorporated herein by reference.

BACKGROUND

Spacers are common features used in the manufacturing of integrated circuits, and are used in both the front end of line processes, which include the formation of transistors, and back end of line processes, which include the formation of the metal connections to connect the transistors. For example, transistors often have gate spacers that are on the gate electrodes of the transistors. The gate spacers electrically insulate the gate electrodes from other conductive features. Furthermore, the gate spacers are used to define the locations of source and drain regions of the transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross-sectional views of intermediate stages in the manufacturing of an integrated circuit structure in accordance with some exemplary embodiments, wherein spacers with rectangular profiles are formed; and

FIG. 9 illustrates a cross-sectional view of a transistor in accordance with some exemplary embodiments, wherein the transistor has gate spacers with rectangular cross-sectional view shapes.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

An integrated circuit structure with rectangular spacers and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages of forming the integrated circuit structures are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 1 illustrates a portion of wafer 100, which includes base layer 20 and patterned features 24 over base layer 20. In some exemplary embodiments, base layer 20 is a dielectric layer. For example, base layer 20 may comprise a low-k dielectric material having a k value lower than about 3.0, which low-k dielectric material may contain carbon, and may have pores therein. In these embodiments, base layer 20 may be resided over semiconductor substrate 25, which may be a silicon substrate, a silicon carbon substrate, a III-V compound semiconductor substrate, or the like. Integrated circuit devices 27 (such as transistors) may be formed at the top surface of semiconductor substrate 25. Patterned features 24 may be dielectric patterns in these embodiments. For example, patterned features 24 may be formed of an oxide, a nitride, or the like. Alternatively, patterned features 24 are formed of amorphous silicon. The formation of patterned features 24 may include forming a blanket layer over base layer 20, and then patterning the blanket layer. Patterned features 24 may have substantially vertical sidewalls.

In alternative embodiments, base layer 20 is a semiconductor substrate, which may be a silicon substrate, a silicon carbon substrate, a III-V compound semiconductor substrate, or the like. In these embodiments, patterned features 24 may be gate stacks of transistors. For example, patterned features 24 may include gate dielectrics 26 and gate electrodes 28 over gate dielectrics 26.

Referring to FIG. 2, spacer layer 30 is formed over the top surfaces and on the sidewalls of patterned features 24. In some embodiments, spacer layer 30 is formed of a dielectric material or metal spacer, which may be an oxide (such as silicon oxide and titanium oxide), a nitride (such as silicon nitride and titanium nitride), or other dielectric materials. Spacer layer 30 may have a homogeneous structure, and hence the entire spacer layer 30, from top to bottom, is formed of the same material. Spacer layer 30 may be formed using a deposition method that is configured to form a conformal layer, which deposition method may be selected from Atomic Layer Deposition (ALD), Chemical Vapor Deposition (CVD), Metal Organic Chemical Vapor Deposition (MOCVD), or the like. The resulting spacer layer 30 hence may be conformal. For example, thickness T1 of the vertical portions is close to thickness T2 of the horizontal portions, with the absolute value of the difference between T1 and T2 being smaller than about 25% percent of both T1 and T2. In some exemplary embodiments, thicknesses T1 and T2 are in the range between about 5 nm and about 500 nm. It is appreciated, however, that the values recited throughout the description are merely examples, and may be changed to different values.

FIG. 3 illustrates the formation of protection layer 32, which forms a blanket layer on the vertical portions and the horizontal portions of spacer layer 30. Protection layer 32 comprises a material different from the material of spacer layer 30. For example, in the embodiments wherein spacer layer 30 is an oxide layer, protection layer may be a nitride-containing layer, and may further include oxygen, or may be free from oxygen. In the embodiments wherein spacer layer 30 is a nitride layer, protection layer may be an oxide-containing layer, and may further include nitrogen, or may be free from nitrogen. The difference between protection layer 32 and the underlying oxide layer 30 is significant enough, so that in subsequent etching processes, appropriate etchants can be selected to attack one of protection layer 32 and the underlying oxide layer 30, but not the other.

Protection layer 32 is formed as a conformal layer, so that the vertical portions and the horizontal portions of protection layer 32 have similar thicknesses. In some embodiments, thickness T3 of protection layer 32 is in the range between about 5 Å and about 50 Å, although greater or smaller thicknesses may be used.

In some exemplary embodiments, protection layer 32 is formed in a process chamber or a furnace, in which a process gas is provided. The process gas may be selected from nitrogen (N₂), oxygen (O₂), a combined gas of CH₄ and nitrogen (N₂), CH₂F₂, or the like, depending on the material of spacer layer 30 and the desirable material of protection layer 32.

The formation of protection layer 32 may include reacting a surface layer of spacer layer 30 with a process gas, so that the surface layer is converted to protection layer 32. For example, when spacer layer 30 comprises an oxide (which may be nitrogen free) such as silicon oxide, a nitrogen-containing gas such as N₂ may be used to perform a nitridation to convert a surface layer of spacer layer 30 to a nitrogen-containing layer. The resulting protection layer 32 includes a nitride of the material of spacer layer 30. For example, protection layer 32 includes silicon oxynitride in some exemplary embodiments. In these embodiments, spacer layer 30 has a silicon-to-oxygen ratio, which is the ratio of the silicon atoms to the number of oxygen atoms, the same as the silicon-to-oxygen ratio in protection layer 32.

When spacer layer 30 comprises a nitride such as silicon nitride (which may be oxygen free), an oxygen-containing gas such as O₂ may be used to perform an oxidation to convert a surface layer of spacer layer 30 to an oxide. The resulting protection layer 32 includes an oxide of the material of spacer layer 30. For example, protection layer 32 includes silicon oxynitride in some exemplary embodiments. In these embodiments, spacer layer 30 has a silicon-to-nitrogen ratio, which is the ratio of the silicon atoms to the number of nitrogen atoms, the same as the silicon-to-nitrogen ratio in protection layer 32. In the embodiments in which protection layer 32 is formed through reaction, the thickness of protection layer 32 is uniform, with the vertical portions and the horizontal portions having the same thickness T3.

The formation of protection layer 32 may be achieved through a deposition process. For example, when the combined gas of CH₄ and N₂ is used, the resulting protection layer 32 may be a carbon and nitrogen (CN) containing layer, which is deposited on the spacer layer 30 in FIG. 2. The CN-containing layer may be a polymer layer. In some other exemplary embodiments in which CH₂F₂ is used as the process gas, the resulting protection layer 32 may be a carbon and fluorine (CF) containing polymer layer, which is deposited on the spacer layer 30 in FIG. 2.

In an exemplary process, in the formation of protection layer 32, wafer 100 may be heated or not heated. For example, the formation of protection layer 32 may be performed at a temperature in the range between about 10° C. and about 500° C. During the formation, plasma may be (or may not be) turned on. When plasma is turned on, the power may be in the range between about 10 watts and about 2,000 watts. There is no bias voltage or substantially no bias voltage applied, so that wafer 100 is not bombarded. When the formation of protection layer 32 is formed in a process chamber, the pressure of the process chamber may be between about 0.1 mtorr and about 50 mtorr. The flow rate of the process gas may be between about 1 sccm and about 2,000 sccm.

Referring to FIG. 4, after the formation of protection layer 32, an anisotropic etching is performed to remove the horizontal portions of protection layer 32, while the vertical portions of protection layer 32 remain not etched. Spacer layer 30 is also not etched. The etching may be a dry etching, and the etchant gas is selected, so that the etching selectivity (referred to a as a first etching selectivity hereinafter) of protection layer 32 and spacer layer 30 is high, wherein the first etching selectivity is the ratio of the etching rate of protection layer 32 to the etching rate of spacer layer 30. For example, the first etching selectivity may be higher than about 4. In the embodiments wherein spacer layer 30 comprises an oxide, while protection layer 32 comprises a nitride, the etchant gas may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y), or combinations thereof. In the embodiments wherein spacer layer 30 comprises a nitride, while protection layer 32 comprises an oxide, the etchant gas may also include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y), or combinations thereof. In the embodiments wherein spacer layer 30 comprises a CN-containing polymer, the etchant gas may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y), or combinations thereof. In the embodiments wherein spacer layer 30 comprises a CF-containing polymer, the etchant gas may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y), or combinations thereof. The etching selectivity may be adjusted by tuning the flow rate ratio of the chemistry used.

After the etching of protection layer 32, spacer layer 30 is patterned. Referring to FIG. 5, the resulting remaining portions of spacer layer 30 and protection layer 32 are referred to as spacers 34 hereinafter. In some embodiments, the etching is performed using an etchant different from the etchant used in the step shown in FIG. 4, which is to etch protection layer 32. The etching may also be an anisotropic etching (with isotropic effect included or not included). In the patterning of spacer layer 30, the horizontal portions of spacer layer 30 are removed, while the vertical portions of spacer layer 30 remain not etched. The etching may be a dry etching, and the etchant gas is selected, so that the etching selectivity (referred to a as a second etching selectivity hereinafter) of spacer layer 30 and protection layer 32 is high, wherein the second etching selectivity is the ratio of the etching rate of spacer layer 30 to the etching rate of protection layer 32. For example, the second etching selectivity may be higher than about 4. In the exemplary embodiments wherein spacer layer 30 comprises an oxide, while protection layer comprises a nitride, the etchant may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y) or other carbon and fluorine containing gas. In the embodiments wherein spacer layer 30 comprises a nitride, while protection layer comprises an oxide, the etchant may also include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y) or other carbon and fluorine containing gas. In the embodiments wherein protection layer 32 comprises a CN-containing polymer, the etchant may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y). In the embodiments wherein protection layer 32 comprises a CF-containing polymer, the etchant may include CF₄, Cl₂, CH₄, N₂, O₂, H₂, C_(x)F_(y).

During the patterning of spacer layer 30, protection layer 32 protects the sidewalls portions of spacer layer 30. As a result, the outer edges of the resulting spacers 34 are substantially vertical, and spacers 34 have a rectangular shape in the cross-sectional view. As a comparison, if protection layer 32 is not adopted, the outer sidewalls of the resulting spacers will be more curved, with the upper portions increasing narrower than lower portions. The rectangular-shaped spacers 34 have a substantially uniform width from top to bottom (with protection layer 32 has a thickness much smaller than the thickness of spacer layer 30).

In some embodiments, for example, when patterned features 24 are mandrels (sacrificial patterns formed for forming spacers 34), patterned features 24 are removed, leaving spacers 34. FIG. 6 illustrates the resulting structure. Hence, the average pitch of spacers 34 is as small as a half of the pitch of patterned features 24. Through the using of mandrels to form spacers, the pitch of spacers 34 may be reduced to values smaller than what the photolithography process can achieve.

FIG. 7 illustrates the etching of base layer 20 using spacers 34 as an etching mask, so that recesses 36 are formed to extend from the top surface of base layer 20 into base layer 20. In these embodiments, base layer 20 may be a dielectric layer, a silicon substrate, or the like. Next, as shown in FIG. 8, features 38, which are formed of a material different from the material of base layer 20, are formed to fill the recesses 36 in FIG. 7. Features 38 are formed of a material different from the material of base layer 20. For example, when base layer 20 is a semiconductor substrate, features 38 may be dielectric features. When base layer 20 is a dielectric layer such as a low-k dielectric layer, features 38 may be metal lines such as copper lines. Spacers 34 shown in FIG. 7 are removed, as shown in FIG. 8.

FIG. 9 illustrates the cross-sectional view of an integrated circuit structure in accordance with alternative embodiments, in which patterned features 24 are gate stacks including gate dielectrics 26 and gate electrodes 28. In these embodiments, spacers 34 are the gate spacers of the resulting transistors 40, which further includes source and drain regions 42 in base layer 20, which is a semiconductor substrate. In the resulting structure, gate spacers 34 include the remaining spacer layers 30 as inner portions, which is homogenous. Protection layer 32 forms the outer portions of gate spacers 34, with each of spacer layers 30 having an inner edge contacting an edge of gate electrode 28, and an outer edge contacting protection layer 32. Protection layer 32 does not have any horizontal portion in the exemplary embodiments.

FIG. 9 also illustrates dielectric layer 44, which may include Contact Etch Stop Layer (CESL) and Inter-Layer Dielectric (ILD) over the CESL. Dielectric layer 44 may be formed of a material different from the material of protection layer 32. For example, dielectric layer 44 may comprise Phospho-Silicate glass (PSG), Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), Tetraethyl Orthosilicate (TEOS) oxide, TiN, SiOC, or other applicable materials. The bottom surfaces of protection layer 32 are higher than, and spaced apart from, the top surface of substrate 20 (and/or source/drain regions 42), with dielectric layer 44 separating the bottom surfaces of protection layers 32 from substrate 20 (and/or source/drain regions 42).

Protection layer 32 forms a vertical thin film that is between spacer layer 30 and dielectric layer 44. From the top end to the bottom end, the thickness of protection layer 32 may be substantially uniform. For example, the lower 90 percent of protection layer 32 has a uniform thickness that has a fluctuation smaller than about 5 percent.

The embodiments of the present disclosure have some advantageous features. By forming protection layer 32 and removing the vertical portions of protection layer 32, spacers 30 may be formed to have a vertical profile. For example, when transferring the patterns of the spacers down to an underlying layer, with the spacers having a rectangular profile, the widths of the underlying features, whose patterned are defined by the spacers, are more uniform.

In accordance with some embodiments, a method includes forming a spacer layer on a top surface and sidewalls of a patterned feature, wherein the patterned feature is overlying a base layer. A protection layer is formed to contact a top surface and a sidewall surface of the spacer layer. The horizontal portions of the protection layer are removed, wherein vertical portions of the protect layer remain after the removal. The spacer layer is etched to remove horizontal portions of the spacer layer, wherein vertical portions of the spacer layer remain to form parts of spacers.

In accordance with other embodiments, a method includes forming a spacer layer on a top surface and sidewalls of a patterned feature, wherein the patterned feature is overlying a base layer, and reacting a surface layer of the spacer layer with a process gas to generate a protection layer. A bottom layer of the spacer layer remains un-reacted with the process gas. Horizontal portions of the protection layer are removed using a first etchant gas, wherein vertical portions of the protect layer remain after the removing. The spacer layer is then etched to remove horizontal portions of the spacer layer using a second etchant gas different from the first etchant gas, wherein vertical portions of the spacer layer remain to form parts of spacers.

In accordance with yet other embodiments, a device includes a semiconductor substrate, a gate stack over the semiconductor substrate, and a gate spacer on a sidewall of the gate stack. The gate stack includes an inner portion having an inner sidewall contacting a sidewall of the gate stack, and an outer portion comprising an inner edge contacting an outer edge of the inner portion. The inner portion and the outer portion include different materials. The out portion has a substantially uniform thickness, and has a bottom surface over and spaced apart from the semiconductor substrate. The device further includes a source/drain region adjacent to the gate spacer.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A device comprising: a semiconductor substrate; a gate stack over the semiconductor substrate; a gate spacer on a sidewall of the gate stack, wherein the gate spacer comprises: an inner portion having an inner sidewall contacting a sidewall of the gate stack; and an outer portion comprising: an inner edge contacting an outer edge of the inner portion, wherein the inner portion and the outer portion are formed of different materials, and wherein the out portion has a substantially uniform thickness; and a bottom surface over and spaced apart from the semiconductor substrate; and a source/drain region adjacent to the gate spacer.
 2. The device of claim 1, wherein the inner portion has a bottom surface contacting a top surface of the semiconductor substrate, and wherein the bottom surface of the outer portion is higher than the bottom surface of the inner portion.
 3. The device of claim 2 further comprising a dielectric layer extending from a level higher than a top surface of the gate spacer to a top surface of the source/drain region, wherein the dielectric layer comprises a bottom portion directly underlying the outer portion of the gate spacer.
 4. The device of claim 3, wherein the bottom portion of the dielectric layer comprises: a top surface contacting the bottom surface of the outer portion of the gate spacer; and a bottom surface in contact with a semiconductor material.
 5. The device of claim 1, wherein the inner portion of the gate spacer comprises an oxide, and the outer portion of the gate spacer comprises nitrogen in addition to the oxide.
 6. The device of claim 1, wherein the inner portion of the gate spacer comprises a nitride, and the outer portion of the gate spacer comprises an oxynitride.
 7. The device of claim 1, wherein the outer portion of the gate spacer comprises a carbon-and-nitrogen-containing polymer.
 8. The device of claim 1, wherein the outer portion of the gate spacer comprises a carbon-and-fluorine-containing polymer.
 9. A device comprising: a substrate; a first gate spacer and a second gate spacer over the substrate; a gate stack between and contacting the first gate spacer and the second gate spacer; a first dielectric layer contacting a sidewall of the first gate spacer; a second dielectric layer comprising: a top portion having a bottom surface contacting a top surface of the first dielectric layer; a middle portion having a sidewall contacting a sidewall of the first dielectric layer; and a bottom portion having a top surface contacting a bottom surface of the first dielectric layer; and a source region and a drain region in the substrate.
 10. The device of claim 9, wherein the second dielectric layer comprises a contact etch stop layer and an inter-layer dielectric over the contact etch stop layer.
 11. The device of claim 9 further comprising a contact plug substantially penetrating through the second dielectric layer.
 12. The device of claim 9, wherein the first gate spacer has a top surface at a first level, and a bottom surface at a second level, and wherein an entirety of the first dielectric layer is between the first level and the second level.
 13. The device of claim 9, wherein the bottom portion of the second dielectric layer further comprises a bottom surface contacting a semiconductor material of the substrate.
 14. The device of claim 9, wherein the first dielectric layer comprises a carbon-and-nitrogen-containing polymer.
 15. The device of claim 9, wherein the first dielectric layer forms a first interface with the first gate spacer, and the bottom portion of the second dielectric layer forms a second interface with the first gate spacer, and the first interface is substantially continuously connected to the second interface.
 16. A device comprising: a semiconductor substrate; a gate spacer having an outer edge and an inner edge, both extending to contact the semiconductor substrate; a gate stack having an edge contacting the inner edge of the gate spacer; a first dielectric layer contacting the outer edge of the gate spacer, wherein the first dielectric layer has a bottom higher than a bottom of the outer edge of the gate spacer; and a second dielectric layer comprising a bottom portion separating the first dielectric layer from the semiconductor substrate.
 17. The device of claim 16, wherein the first dielectric layer has a lengthwise direction parallel to the outer edge of the gate spacer in a cross-sectional view of the device, and wherein a top edge of the first dielectric layer is not higher than a top edge of the gate spacer, and a bottom edge of the first dielectric layer is higher than a bottom edge of the gate spacer.
 18. The device of claim 16, wherein the first dielectric layer is in contact with the outer edge of the gate spacer.
 19. The device of claim 16, wherein the second dielectric layer comprises: a top portion having a bottom surface contacting a top surface of the first dielectric layer; a middle portion having a sidewall contacting a sidewall of the first dielectric layer; and the bottom portion having a top surface contacting a bottom surface of the first dielectric layer.
 20. The device of claim 19, wherein the bottom portion of the second dielectric layer is overlapped by the first dielectric layer. 